Jean-Pierre Bouillin a Jean-Luc Bouchez b p. Lespinasse b and A. P~cher a .... King [36] and Rousset and Daly [37], among others. Since then, AMS has been ...
Earth and Planetary Science Letters, 118 (1993) 263-279 Elsevier Science Publishers B.V., Amsterdam
263
[PT]
Granite emplacement in an extensional setting: an AMS study of the magmatic structures of Monte Capanne (Elba, Italy) Jean-Pierre Bouillin
a Jean-Luc
Bouchez
b p.
Lespinasse b and A. P~cher a
Laboratoire de G~odynamique alpine, CNRS URA 69, Universitd Joseph Fourier, rue M. Gignoux, 38031 Grenoble Cedex, France b Laboratoire de P~trophysique, CNRS URA 67, Universitd Paul-Sabatier, 38 rue des 36-Ponts, 31400 Toulouse Cedex, France Received August 24, 1992; revision accepted May 14, 1993
ABSTRACT Monte Capanne, Elba is an excellent example of a granite pluton (42 km 2) emplaced 6 m.y. ago in a well-known extensional environment, that of the Tyrrhenian Sea. Its internal fabric has been studied using the magnitude of magnetic susceptibility and anisotropy measurements at 74 regularly distributed stations. Susceptibility magnitudes reveal a normal and asymmetrical petrographical zoning in the pluton. The magnetic lineations are remarkably well organized, mostly of NW-SE azimuth with slight plunges, and always normal to the walls of the late aplitic dykes. These lineations are also parallel to the stretching lineations of the immediate country rocks of the pluton. In the western and probably deepest part of the pluton a NE-SW trending magnetic/magmatic lineation is also imprinted. The following scenario is proposed for the emplacement of Monte Capanne during the opening of the Tyrrhenian Sea: An approximately E - W trending sinistral transform fault is taken as being responsible for the creation of a pull-apart void that permitted upwelling of the magma. Orientations of the magmatic structures at depth are parallel to the elongation of the feeding zone of this void, i.e. NE-SW. The very top part of the pluton was sheared parallel to the granite-country rock interface, i.e. NW-SE, parallel to the local stretching of the Tyrrhenian Sea at the time of emplacement.
1. Introduction The Monte Capanne granodiorite, one of the youngest plutons outcropping on Earth, has been closely studied using isotope chronology. As its geological environment and tectonic context are well documented, both above and below sea level, it constitutes an excellent target for a structural study of the uppermost crustal emplacement of a magma. The aim of this paper is to document the pattern of the internal fabric of this pluton by means of the anisotropy of magnetic susceptibility (AMS) technique, and thus to obtain some clues about the way magma can pierce the upper brittle crust. 2. General setting Monte Capanne, which lies at the westernmost extremity of Elba (Fig. 1), is a granodioritic plu-
ton belonging to the Tuscany Magmatic Province, characterized by an eastward migration of its magmatic activity from Late Miocene to Quaternary times [1-5]. Geodynamically this magmatism has been interpreted in various ways: Marinelli [2] suggested that it was a hotspot; Alvarez [6] and Reutter et al. [7] proposed that the magmatism was due to an eastward subduction of the Corsica-Sardinia microplate. Now, howver, many authors, except Wezel [8], agree that the magmatism of Tuscany is related to the extension of the Tyrrhenian Sea, via the westward subduction of the Adriatic plate. The Tyrrhenian Sea would therefore be a back-arc basin [9-11] or a more complex basin related to the movements of the Adriatic slab [12]. The Monte Capanne pluton intrudes into and affects by contact metamorphism ophiolitic and sedimentary series belonging to the Ligurian Nappes of the Northern Apennines. Many iso-
0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
264
J'-P" B O U I L L I N El" AL.
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lenian F i g . 1. M o n t e
Capanne
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t r a n s f o r m f a u l t [ a f t e r 11].
tope chronology studies using various techniques have been performed, including K/Ar, Rb/Sr, U / P b and fission track [13-21]. Whole-rock Rb/Sr isochrons give ages close to 7 Ma while Rb/Sr on mineral fractions and U / P b provide age dusters around 6.2 Ma (Fig. 2). These ages indicate that Monte Capanne is younger than the compressional events of the internal, western
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3. Magnetic susceptibility data of the pluton Because the internal anisotropy of the granite is too low in intensity for precise orientation measurements, especially of the linear fabrics (both in the field with a compass and in the laboratory under the microscope), we decided to explore the anisotropy of magnetic susceptibility of the pluton. This technique, which has been reviewed by Hrouda [33] and Borradaile [34], was proposed as a promising tool for structural geology by Graham [35] and applied to granites by King [36] and Rousset and Daly [37], among others. Since then, AMS has been applied to many plutons for kinematic reconstruction of their emplacement [38-44].
/ Monte Capanne Pluton
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266
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GRANITE
EMPLACEMENT
IN AN EXTENSIONAL
SE'lq'ING AND MAGMATIC
Kmin). Results for each pair of specimens from
the same core are usually very close, both in magnitude and orientation. Larger variations generally occur between two cores of the same sampling station, especially in the AMS orientations, but the discrepancies often remain moderate in magnitude, as is shown in the following sections.
3.2 Magnetic susceptibility magnitudes Table 1 gives the bulk magnitude ( K in 10 -5 SI) of the magnetic susceptibility at each station, calculated as the arithmetic average of the four specimens. The m e a n magnetic susceptibility for the whole pluton is K = 14.0 X 10 -5 SI. MS magnitudes appear in the frequency histogram of Fig. 4, which shows that all but eight cores range from 1 0. to 18 X 10 - s SI, and that 76% of the cores range from 12 to 16 x 10 -5 SI. The only station which gives a very low K ( = 5 X 10 -5 SI) (IE 48) lies on an aplitic dyke. In contrast, K values higher than 20 x 10 - s SI are exceptional, and are due to very local mineralogical heterogeneities
STRUCTURES
ON ELBA
267
(stations I E 17a, IE 64a and IE 67a), as the second rock cylinder of each station gives a significantly lower K value (IE 17b, IE 64b and IE 67b; Table 1). In spite of the low amplitude of its variation over the pluton, the magnetic susceptibility magnitude is well organized on the map (Fig. 4). The eastern side of the pluton, i.e. to the east of the Capo San A n d r e a - F e t o v a i a line, shows susceptibility magnitudes increasing toward the contact with the country rocks. This gradation contrasts with the western half of the pluton, where lower susceptibility magnitudes show no clear gradation toward the contact. This suggests that the granite and its country rocks have shared a common history during emplacement in the east, and that the granite was decoupled from its host in the west.
3.3 Anisotropy of magnetic susceptibility data Table 1 gives averages of the AMS data at each of the 74 stations: vectorial averages of the Kma x and Kmi n orientations, and arithmetic aver-
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269
G R A N I T E E M P L A C E M E N T IN A N E X T E N S I O N A L S E T T I N G A N D M A G M A T I C S T R U C T U R E S ON E L B A
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J.-P. BOUILLIN ET AL.
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